This invention relates to fuel control systems for gas turbine engines. One example of its use is in engines for aircraft capable of landing vertically.
The British Aerospace P.L.C. Harrier and the McAir AV8A and AV8B aircraft are powered by a Rolls-Royce PLC Pegasus engine which employs vectorable hot and cold propulsive thrust nozzles. When landing vertically the nozzles are pointed downwards and the fuel flow is adjusted to control the upward thrust. By adjusting the upward thrust, vertical speed of the aircraft can be changed and the aircraft can be landed in a controlled manner. At the instant that the undercarriage wheels contact a firm base, some of the weight of the aircraft is transferred to the wheels and consequently the upward thrust momentarily exceeds the weight of the aircraft. Unless this thrust is diminished rapidly the aircraft is prone to "jump" upwards causing problems, for example, on wet decks. During descent, pilots are trained to detect "touch-down" and slam the throttles shut immediately. If the pilot or the fuel control system responds too slowly, the aircraft may bounce.
One current form of fuel control system employs a digital electronic control unit (DECU) which monitors various engine parameters (such as pressures, temperatures, speeds of rotation of spools etc.) and is responsive to movements of the pilot's throttle lever, to operate a motor driven metering valve which controls the flow of fuel to the engine. However, the response of such a fuel control system when a pilot slams the throttles shut on touch down can sometimes be slow, because of factors such as the ramp time taken by the motor which drives the metering valve to produce a corresponding decrease in fuel flow.
There are also other situations in which a rapid reduction in fuel flow may be required. For example immediately after an engine surge has been detected, a sudden decrease of fuel can enable the surge to be controlled.